Flocculation steric stabilisation

Vincent, B., Edwards, J., Emmett, S. and Jones, A. (1986) Depletion flocculation in dispersions of sterically-stabilised particles (soft spheres). Colloids Surf,... 

Table 8.4 Classification of sterically stabilised dispersions and comparison of critical flocculation temperatures (c.f.t) with theta-temperatures112 (By courtesy of Academic Press Inc.)...

The sterically stabilised dispersions produced can be weeikly flocculated by the addition of free (non-adsorbing) polymer such as poly(ethylene oxide). 

Non-Aqueous Processes. Dispersions of composite particles in non-aqueous media (12) have been prepared. The particles were sterically stabilised to prevent flocculation and aggregation. This was achieved by physical absorption of amphipathic graft or block copolymer (13,14) or by covalent attachment of diluent-soluble oligomer or polymer chains (15) at the particle surface so that by definition different polymers were situated at the surface and in the bulk of the particles, even for single-polymer particles. Composite particles were prepared by slow addition of the second monomer which was fully miscible with the diluent phase, obviating a monomer droplet phase further monomer-soluble initiation and amphipathic graft stabiliser was included as appropriate so that the process comprised continued dispersion... 

These authors studied the steric stabilisation of aqueous BaTiOs suspensions with block copolymers, which fulfil both the stabilising and binder function. Some block copolymers with PVA and polyacrylic acid blocks were found to be very suitable for this purpose. They found further that depletion flocculation occurs with random copolymers. In that case the homogeneity of dried layers prepared was lower. 

The small droplets have much smaller van der Waals attraction, and flocculation is prevented. This is particularly the case with sterically stabilised systems. 

When >0.5, becomes negative (attractive) this, combined with the van der Waals attraction at this separation distance, produces a deep minimum causing flocculation. In most cases, there is a correlation between the critical flocculation point and the 0-condition of the medium. A good correlation is found in many cases between the critical flocculation temperature (CFT) and the 0-temperature of the polymer in solution (with both block and graft copolymers the 0-temperature of the stabilising chains A should be considered) [2]. A good correlation was also found between the critical volume fraction (CFV) of a nonsolvent for the polymer chains and their 0-point under these conditions. In some cases, however, such correlation may break down, and this is particularly the case for polymers that adsorb by multipoint attachment. This situation has been described by Napper [2], who referred to it as enhanced steric stabilisation. 

This can occur if the energy barrier is small or absent (for electrostatically stabilised emulsions) or when the stabilising chains reach poor solvency (for sterically stabilised emulsions, that is if />0.5). For convenience, the flocculation of electrostatically and sterically stabilised emulsions will be discussed separately. 

Equation (12.10) also shows that when > 0.5 - that is, when the solvency of the medium for the chains becomes poor - Gj j will be negative and the interaction will become attractive. Thus, it is important to ensure that the solvent used to prepare the W/O emulsion is a good solvent for the PHS chains, otherwise flocculation of the water droplets (perhaps followed by their coalescence) may occur. Fortunately, the PHS chains are soluble in most hydrocarbon solvents used in most formulations. The condition = 0.5 is referred to as 0-solvent, and this denotes the onset of a change from repulsion to attraction. Thus, to ensure steric stabilisation by the above mechanism it must be ensured that the chains are kept in better than 0-solvent. 

With a sterically stabilised dispersion, weak flocculation can also occur when the thickness of the adsorbed layer decreases. Again, the value of E can be used as a measure of the flocculation the higher the value of E, the stronger the flocculation. 

At this point we see qualitatively that the mechanisms of steric stabilisation and depletion flocculation are closely related. In the former instance the concentration of polymer segments in the space between the particles increases as the particles come together, leading to a repulsion caused by the osmotic flow of solvent into this space in the latter case the concentration between the particles is lower than that in the bulk, and diffusion of solvent out of the interparticle space results in an attraction. 

Fig. 3.13. Flocculation and stabilisation of suspensions by neutral polymers, (a) Lone particles. The range of electrostatic repulsion has been represented dotted line), (b) When a small quantity of polymer is added, the adsorbed layer remains thin since the chain conformation is highly flattened, (c) When the quantity of polymer is such that the size of loops becomes greater than twice the range of electrostatic repulsion, links can be established between particles. Aggregates form in the solution and precipitate out. (d) The particles are saturated with polymer and repel each other via steric effects, (e) The saturation of particle surfaces by polymers with low molecular weight can favour stability, without increasing the risk of flocculation...

In practice, therefore, the objective is to achieve an intermediate form by the addition of a controlled amount of electrolyte or surfactant. When the particles strongly repel each other, an electrolyte can be added. By decreasing the zeta-potential, the repulsive forces will decrease. When the particles attract each other too strongly a surfactant can be added. As the lyophobic part of the surfactant molecule adsorbs onto the surface of lyophobic colloids its lyophilic part will be oriented into the dispersion medium. By steric stabilisation, the attraction forces are decreased. The properties of flocculated and deflocculated suspensions are summarised in Table 18.18. 

As will be described in the section on flocculation, the total energy-distance of separation curve for electrostatically stabilised shows a shallow minimum (secondary minimum) at relatively long separation between the droplets. By addition of small amounts of electrolyte such minimum can be made sufficiently deep for weak flocculation to occur. The same applies for sterically stabilised emulsions, which show only one minimum, whose depth can be controlled by reducing the thickness of the adsorbed layer. This can be achieved by reducing the molecular weight of the stabiliser and/or addition of a non-solvent for the chains (e.g. electrolyte). 

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